Many telecommunication customer features require the function of being able to route a call intended for one telecommunication terminal to another telecommunication terminal without human interaction. To illustrate this need consider "send all calls" and "call coverage" features that are standard on most telecommunications systems such as the AT&T Definity.RTM. Business Communications Systems. The "send all calls" feature operates by rerouting any call intended for a first telephone station set to a second telephone station set. There are two variations of the "send all calls" feature. The first variation is where the user of the first station set manually designates the telephone number of the second telephone station set by performing certain operations on the first telephone station set. The second variation of the "send all calls" feature is where the telephone number of the second station set is administered by the system administrator of the telecommunication switching system. The "call coverage" feature is similar to the "send all calls" with the exception that the "call coverage" feature is utilized in what is normally called the "boss-secretary" relationship. The " call coverage" normally involves a number of station sets in a call coverage group. Each station set in the call coverage group can receive a rerouted call from the intended telephone station set (also referred to as the covered station set). Coveting telephone station sets are arranged in a hierarchy for each telephone station set being covered with the call being attempted to be rerouted starting at the top of the hierarchy. The purpose of the hierarchy is to assure that the rerouted telephone call is answered by the person having the greatest knowledge of how to handle the call.
Prior art systems have implemented these features in one of two ways. The first method is to have the central processor controlling the telecommunications switching system maintain in memory information pertaining to telephone station sets invoking these features and the covering telephone station sets. This method is disclosed in U.S. Pat. Nos. 4,682,354 and 4,790,004. For example, for the "send all calls" feature, the central processor is responsive to the user of a telephone station set actuating a button on the telephone station set to record in the memory of the central processor the actuation of the "send all calls" feature. Each time a call is directed to a telephone station set, the central processor checks a record memory to determine whether or not the "send all calls" feature has been activated for that particular station set. If the "send all calls" feature has been activated, the central processor obtains the identity of the covering station set and routes the call to that station set. The identity of the covering station set was either entered by the user of the covered station set or by the system administrator. The "call coverage" feature is executed in a similar manner, but the central processor has to maintain information in memory concerning the call coverage groups. In addition, it is important that the central processor identify for the coveting station set the telephone number of the covered telephone station set or the name of the user of the covered station set.
Other prior art telecommunication switching systems have provided for the routing of telephone calls by having the covered telephone station set accept the call, place a second call to the covering telephone station set, and then signal the telecommunication system to transfer the first telephone call to the covering telephone station set. One prior art system using this method of rerouting telephone calls is the SX 2000 telecommunication system manufactured by Mitel Inc. In addition, this type of call rerouting is described in U.S. Pat. No. 5,036,535 for use in automatic call distribution (ACD) systems. This method of using "call transfer" to reroute the call does not forward the calling telephone number to the covering telephone station set and does not allow the call to be answered in an informative manner since the person using the covering telephone doesn't know who is calling.
Whereas the two previously described methods of rerouting telephone calls have provided this function for many years, these two methods suffer nevertheless from problems which become more severe as more modern telecommunication architectures come into existence. The first method of rerouting calls suffers from the problem that the central processor must maintain extensive records with respect to all telephone station sets connected to the telecommunication switching system. In addition to the maintenance of these records, the central processor has to be responsive to actuations of various buttons on each of the telephone station sets to record the features invoked by those buttons. This results in the telecommunication system having to maintain extensive records of what each button signifies on each individual telephone station set. This information has to be entered by the system administrator of the telecommunication switching system and is subject to error in addition to being a costly process to perform. The second method overcomes some of the problems of the first method but suffers from problems of its own. The main problem of the second method is the requirement of placing the first telephone call on hold and then performing a call transfer between the first and second calls. This places a load on the telecommunication switching system and is limited to the functions performed by the "call transfer" feature provided by the telecommunication switching system. One such limitation imposed by the transfer feature is that the telephone number of covered telephone set is not communicated by the covering telephone station set.